PET is a method that includes administering, into a living body, a tracer labeled with a short-term radionuclide which releases positrons such as 18F or 11C so that γ rays generated from the tracer is measured by a PET camera (detector comprising a gamma ray scintillator and a photoelectron multiplier tube), and imaging a body distribution of the γ ray by a computer. The PET can non-invasively and quantitatively track down the movement of materials over time in vivo, therefore, is now actively employed as a very useful measurement technique in many different fields such as biology, development of pharmaceutical products, and medical services.
Examples of the short-term radionuclide used in the PET are 18F and 11C, and compounds labeled with these radionuclides are used as the tracer. There is a very broad application range for the 11C utilizing carbon atoms present in organic compounds, therefore, 11C can be considered to be an ideal radionuclide. However, the 11C has such a short half life as 20 minutes, imposing the restriction that the process from synthesis through PET measurement must be completed within a very short time frame. In contrast, the 18F having a half life longer than that of the 11C, 110 minutes, is easy to handle and widely used in, for example, 18F-labeled glucose. Despite such an advantageous half life, positrons released by the 18F decrease with time, making the PET measurement similarly difficult. Faced with such a difficulty, a fast and simplified 18F labeling method is desirably accomplished.
To prepare an 18F-labeled compound, its base material, 18F, is supplied through nuclide transformation by a cyclotron. More specifically, ions accelerated by the cyclotron are crashed into water including 18O to invite nuclide transformation from 18O to 18F. The 18F-labeled compound can be theoretically obtained when a very weak water solution including 18F ions thus obtained and a compound selected as a PET target are reacted with each other.
In fact, it is conventionally extremely difficult to directly bond 18F to the compound for the PET. Therefore, an indirect 18F labeling method is employed, wherein an intermediate compound labeled with 18F and having a functional group to be bonded to another compound (intermediate medium thus characterized is hereinafter called “18F prosthetic group”) is prepared in advance and then bonded to the compound for the PET.
Examples of properties required for the 18F prosthetic group are: 1) easily and speedily synthesizable; 2) bondable to a target compound rapidly and efficiently under temperate conditions; and 3) hardly affecting in-vivo kinetics of the target compound. To meet these requirements, an 18F prosthetic group which leverages the Huisgen reaction is currently attracting attention.
Describing the Huisgen reaction, an organic azide compound and alkynes, as a result of a [3+2] cycloaddition reaction generated therebetween, are transformed into a 1,2,3-triasole derivative (see the following chemical formula).

In the case where a terminal alkyne is used as the alkynes, copper (I) ions work as a catalyst, and a 1,4-disubstituted body is selectively obtained. This reaction is not disturbed by any other functional groups, if present, such as alcohol, amine, amide, ester, and halide, therefore, a target material, triaosle, can be obtained at a high yield. The reaction uneventfully advances in any of reaction solvents such as alcohol, conventional organic solvents, and water. The asides and alkynes have a number of advantages; they can be easily introduced in various organic compounds, and they do not produce excessive post-reaction waste matters. Therefore, the asides and alkynes are cited as typical examples which represent the Click Chemistry advocated by K. B. Sharpless et al. who was awarded with Nobel Prize in Chemistry.
In the studies conducted in recent years using the Huisgen reaction, azide compounds or alkyne compounds labeled with 18F are used as the 18F prosthetic group to introduce the 18F prosthetic group in any target compounds. Below are given specific examples of such studies.
Marik and Sutcliffe, University of California, Davis, fluorinated a tosylate body having an acetylene group on its molecular end in the presence of [18F]KF and kalium scavenger (Kryptofi x222) and purified through distillation so that 18F-labeled fluoroalkynes are obtained as the 18F prosthetic group (Non-Patent Document 1). Then, they generated the Huisgen reaction between the 18F prosthetic group and azidated peptide in the presence of a catalytic system including iodinated copper (I), sodium ascorbate, and di-isopropyl ethylamine as amine base to perform 18F-labeling of the peptide at room temperature for 10 minutes (see the following reaction formula).

They further performed 18F-labeling of αvβ6 specific peptide using the 18F prosthetic group in which n=3 in the above formula, and successfully obtained PET images in mice in vivo (Non-Patent Document 2).
Another study using the 18F prosthetic group in which n=1 in the above formula was also reported by a study group of Sungkyunkwan University, Korea (Non-Patent Document 3).
A study group representing SIEMENS MEDICAL SOLUTIONS USA, INC. reported the use of a compound comparable to n=0 in the above formula (Patent Document 1), wherein propargyl tosylate is 18F-transformed to generate an 18F prosthetic group ([18F]-3-fluoropropyne) in a reaction container and leave the generated 18F prosthetic group unpurified, and an overly large quantity of azide substrate is added to a reaction solution to generate a coupling reaction using copper (I) acetate.
The Non-Patent Document 4 recites an example in which an 18F-labeled azide compound is used, wherein the Huisgen reaction is generated with oligopeptide having a terminal acetylene group by using [18F] fluoroethylene azide as 18F so that the oligopeptide is successfully labeled with 18F (see the following chemical formula).

A study group of Inha University, Korea reported the use of a plurality of 18F-labeled alkyne compounds and 18F-labeled azide compounds (see the following chemical formula) as the 18F prosthetic group, wherein they performed 18F-labeling of a variety of substrates (see the Non-Patent Document 5). However, their method directly performing the coupling reaction without purifying the 18F prosthetic group similarly uses an overly large quantity of compounds to be 18F-labeled as the 18F prosthetic group.

A study group of Stanford University purified the 18F prosthetic group by employing preparative HPLC more easily automatable than distillation to thereby obtain an 18F-labeled alkyne compound having a high purity (Non-Patent Document 6). They generated the Huisgen reaction using a catalytic system including copper sulfate—sodium ascorbate but no amine base to label ligands of integrin αvβ3 with 18F and captured PET images in mice.
A study group of Dresden-Rossendor Research Center, Germany purified 4-[18F]Fluoro-N-(prop-2-ynyl)benzamide synthesized by the following method by employing SPE (solid phase extraction) and used the purified material as the 18F prosthetic group, and then performed the 18F-labeling of azidated Neurotensin (8-13) peptide through the Huisgen reaction (Non-Patent Document 7).

They discussed the conditions of the Huisgen reaction to reduce the quantity of peptide used in each 18F-labeling test, and found out that an optimal condition is to use the catalytic system including copper sulfate and sodium ascorbate with a borate buffer, and finally succeeded in reducing the quantity of azidated Neurotensin (8-13) peptide to 1 mg (approximately 1 μmol). However, it is necessary to further reduce the quantity for practical use.
A study group of TRIUMF, Canada, based on their hypothesis that aryl fluorine is probably more metabolically stable than aliphatic fluorine, developed an aryl fluorine 18F prosthetic group synthesizable in one stage, which was reported in 2008 (Non-Patent Document 8).
More specifically describing their study, a position 2 nitro group or trimethyl ammonio group of the pyridine derivative expressed by the following chemical formula is fluorinated and purified by the HPLC so that an 18F prosthetic group having a high purity is obtained, and the 18F prosthetic group was subjected to the Huisgen reaction with a compound to be 18F-labeled (azidated peptide precursor) in the presence of a catalytic system including TBTA, Cu (CH3CN)4PF6 and di-isopropyl ethylamine. The quantity of the compound to be 18F-labeled used in their study was, however, 1,400 nmol. This is still a large quantity which needs to be reduced.

In recent years, the gene therapy based on such phenomena as antisense, antigene, decoy, and RNA interference is increasingly progressing. There are ongoing approaches for the gene therapy using natural nucleic acids (DNA, RNA) or artificial nucleic acids having better pharmacological kinetics and physiological activities than natural nucleic acids (DNA, RNA) (for example, 2′-0-MeRNA, phosphorothioate oligo, BNAs, LNA). When it succeeds to label the oligonucleotide of any natural or artificial nucleic acid with 18F and administer the 18F-labeled oligonucleotide to a human or an experimental animal as a PET probe to measure intra-body kinetics using a PET camera, studies and researches of oligonucleotides are expected to further advance, accelerating the development of pharmaceutical products in which the technique is leveraged. Further, once the “double strand with complementary strand”, which is a characteristic phenomenon of oligonucleotides, can be observed in vivo by using the 18F-labeled oligonucleotide (in vivo hybridization), it is facilitated to measure an in-vivo mRNA expression level. This technique is broadly applicable to diagnostics of diseases, and studies and researches of medicine and pharmacy. To meet the needs described so far, the oligonucleotide 18F-labeling method was so far often studied and reported (for example, Non-Patent Documents 9 to 14).